1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and an electrophotographic apparatus.
2. Description of the Related Art
An electrophotographic apparatus includes an electrophotographic photosensitive member with a surface on which a latent image (electrostatic latent image) and a toner image are formed. The electrophotographic photosensitive member needs to offer high-quality and stable electrophotographic characteristics (the electrophotographic characteristics include potential characteristics, such as charging ability, sensitivity (photosensitivity) and residual potential, and image characteristics, such as resolution and gradation) and durability (wear resistance, printing durability, environment resistance and chemical resistance). An electrophotographic photosensitive member commonly used includes a cylindrical substrate (hereinafter simply referred to as a “substrate”) and a photoconductive layer formed on the substrate. A common substrate is formed of metal (alloy) such as aluminum.
A photoconductive material forming the photoconductive layer needs to offer a high sensitivity, a high SN ratio (photo current (Ip)/dark current (Id)), an absorption spectrum compatible with the spectral characteristics of electromagnetic waves (imagewise exposure light) to which the electrophotographic photosensitive member is exposed, high optical responsivity, and a desired dark resistance value.
A photoconductive material that is excellent in these properties is amorphous silicon containing hydrogen atoms (an amorphous material containing silicon atoms as a base and hydrogen atoms). Such amorphous silicon has been put to practical use as a material forming the photoconductive layer of the electrophotographic photosensitive member. The amorphous silicon containing hydrogen atoms is hereinafter also referred to as “a-Si”.
A photoconductive layer made of a-Si (hereinafter referred to as an “a-Si photoconductive layer” or simply a “photoconductive layer”) is generally formed on a substrate heated to 50 to 400° C., by one of a vacuum deposition method, a sputtering method, a thermal CVD method, a photo CVD method, and a plasma CVD method. Among these film formation methods, the plasma CVD method is suitably used, in which a material gas (material substance) is decomposed by high-frequency or microwave glow discharge to form a-Si deposited film on the substrate. Moreover, a surface layer provided with durability against wear and a usage environment involving temperature and humidity is formed on the a-Si photoconductive layer formed as described above. As a result, an electrophotographic photosensitive member suitable for practical use is manufactured.
A significantly increasing number of recent electrophotographic apparatuses can provide color images (full-color images). This has led to the need to allow the electrophotographic photosensitive member to be mounted in full-color electrophotographic apparatuses. An electrophotographic apparatus configured to output color images frequently outputs not only text documents but also photographs, pictures and design pictures. Hence, there has been a stronger demand to improve resolution and suppress image density non-uniformity and image defects than in the case of black and white electrophotographic apparatuses.
In particular, a demand to suppress image density non-uniformity has been stronger year by year. For example, in a halftone area corresponding to a blue sky portion of a landscape photograph, even slight density non-uniformity is very noticeable. Thus, there has been a greater need to suppress image density non-uniformity than before.
However, when an a-Si photoconductive layer is formed by any of the above-described common film formation methods, the substrate needs to be heated to 50 to 400° C. as described above. When an a-Si photoconductive layer is thus formed on the substrate, the ends of the substrate may be deformed to cause end deformation of the electrophotographic photosensitive member.
A major factor causing such deformation of the ends of the electrophotographic photosensitive member is expected to be a difference in the coefficient of thermal expansion between the substrate and the a-Si photoconductive layer. That is, if the substrate and the a-Si photoconductive layer are cooled after the a-Si photoconductive layer is formed on the substrate, different amounts of heat shrinkage are expected to occur in the substrate and the a-Si photoconductive layer owing to the difference in the coefficient of thermal expansion. As a result, stress acts between the substrate and the a-Si photoconductive layer. Such stress is generally higher at the ends of the substrate, which generally involve the largest amount of heat shrinkage. Hence, the ends of the substrate are deformed to cause the end deformation of the electrophotographic photosensitive member. If the ends of the substrate are unlikely to be deformed, internal stress occurs in the a-Si deposited film of the photoconductive layer. Then, the film (a-Si photoconductive layer) may be peeled off at the ends of the photoconductive layer.
The end deformation of the substrate causes the ends of the electrophotographic photosensitive member to be deformed. This reduces the level of the dimensional accuracy of the electrophotographic photosensitive member. Thus, for example, in a system adopting a contact development scheme, a surface layer of the photoconductive layer is likely to be worn away in a portion where the electrophotographic photosensitive member contacts a developing member. This reduces development uniformity (causes development non-uniformity), leading to image density non-uniformity. Furthermore, for example, in a system adopting a non-contact charging scheme such as a corona charger, the level of dimensional accuracy of the electrophotographic photosensitive member may decrease to vary the distance between the charger and the electrophotographic photosensitive member. This reduces charging uniformity (causes charging non-uniformity), resulting in image density non-uniformity.
Japanese Patent Application Laid-Open No. 2007-179025 discloses a method in which a stress relaxing section configured to relax stress acting between the substrate and the photoconductive layer is provided in a non-latent-image-formation area in order to suppress the end deformation of the substrate and the film peel-off at the ends of the electrophotographic photosensitive member.
Furthermore, Japanese Patent Application Laid-Open No. 2004-354967 discloses a method of improving the shape of ends of the substrate to suppress the deformation.
However, in these conventional techniques, the stress relaxing section is provided after formation of the a-Si deposited film. This leads to the need for mechanical processing such as cutting of the electrophotographic photosensitive member, chemical treatment such as etching treatment, and processing of the substrate. Thus, the manufacturing costs of the electrophotographic photosensitive member are likely to increase. Furthermore, the above-described conventional techniques can restrain the film from being peeled off over time when the electrophotographic photosensitive member is used to form images but fail to sufficiently suppress the end deformation of the electrophotographic photosensitive member resulting from a variation in temperature.